DESCRIPTION

This module implements a simple RPC protocol and backend for processes created via AnyEvent::Fork or AnyEvent::Fork::Remote, allowing you to call a function in the child process and receive its return values (up to 4GB serialised).

It implements two different backends: a synchronous one that works like a normal function call, and an asynchronous one that can run multiple jobs concurrently in the child, using AnyEvent.

It also implements an asynchronous event mechanism from the child to the parent, that could be used for progress indications or other information.

EXAMPLES

Example 1: Synchronous Backend

Here is a simple example that implements a backend that executes unlink and rmdir calls, and reports their status back. It also reports the number of requests it has processed every three requests, which is clearly silly, but illustrates the use of events.

The parent creates the process, queues a few rmdir's. It then forgets about the $rpc object, so that the child exits after it has handled the requests, and then it waits till the requests have been handled.

The child is implemented using a separate module, MyWorker, shown here:

The run function first sends a "progress" event every three calls, and then executes rmdir or unlink, depending on the first parameter (or dies with a fatal error - obviously, you must never let this happen :).

Eventually it returns the status value true if the command was successful, or the status value 0 and the stringified error message.

On my system, running the first code fragment with the given MyWorker.pm in the current directory yields:

/tmp/somepath/1: No such file or directory
/tmp/somepath/2: No such file or directory
3 requests handled
/tmp/somepath/3: No such file or directory
/tmp/somepath/4: No such file or directory
/tmp/somepath/5: No such file or directory
6 requests handled
/tmp/somepath/6: No such file or directory

Obviously, none of the directories I am trying to delete even exist. Also, the events and responses are processed in exactly the same order as they were created in the child, which is true for both synchronous and asynchronous backends.

Note that the parentheses in the call to AnyEvent::Fork::RPC::event are not optional. That is because the function isn't defined when the code is compiled. You can make sure it is visible by pre-loading the correct backend module in the call to require:

->require ("AnyEvent::Fork::RPC::Sync", "MyWorker")

Since the backend module declares the event function, loading it first ensures that perl will correctly interpret calls to it.

And as a final remark, there is a fine module on CPAN that can asynchronously rmdir and unlink and a lot more, and more efficiently than this example, namely IO::AIO.

Example 1a: the same with the asynchronous backend

This example only shows what needs to be changed to use the async backend instead. Doing this is not very useful, the purpose of this example is to show the minimum amount of change that is required to go from the synchronous to the asynchronous backend.

To use the async backend in the previous example, you need to add the async parameter to the AnyEvent::Fork::RPC::run call:

->AnyEvent::Fork::RPC::run ("MyWorker::run",
async => 1,
...

And since the function call protocol is now changed, you need to adopt MyWorker::run to the async API.

First, you need to accept the extra initial $done callback:

sub run {
my ($done, $cmd, $path) = @_;

And since a response is now generated when $done is called, as opposed to when the function returns, we need to call the $done function with the status:

$done->($status or (0, "$!"));

A few remarks are in order. First, it's quite pointless to use the async backend for this example - but it is possible. Second, you can call $done before or after returning from the function. Third, having both returned from the function and having called the $done callback, the child process may exit at any time, so you should call $done only when you really are done.

Example 2: Asynchronous Backend

This example implements multiple count-downs in the child, using AnyEvent timers. While this is a bit silly (one could use timers in the parent just as well), it illustrates the ability to use AnyEvent in the child and the fact that responses can arrive in a different order then the requests.

It also shows how to embed the actual child code into a __DATA__ section, so it doesn't need any external files at all.

And when your parent process is often busy, and you have stricter timing requirements, then running timers in a child process suddenly doesn't look so silly anymore.

The parent part (the one before the __DATA__ section) isn't very different from the earlier examples. It sets async mode, preloads the backend module (so the AnyEvent::Fork::RPC::event function is declared), uses a slightly different on_event handler (which we use simply for logging purposes) and then, instead of loading a module with the actual worker code, it eval's the code from the data section in the child process.

It then starts three countdowns, from 3 to 1 seconds downwards, destroys the rpc object so the example finishes eventually, and then just waits for the stuff to trickle in.

The worker code uses the event function to log some progress messages, but mostly just creates a recurring one-second timer.

The timer callback increments a counter, logs a message, and eventually, when the count has been reached, calls the finish callback.

On my system, this results in the following output. Since all timers fire at roughly the same time, the actual order isn't guaranteed, but the order shown is very likely what you would get, too.

While the overall ordering isn't guaranteed, the async backend still guarantees that events and responses are delivered to the parent process in the exact same ordering as they were generated in the child process.

And unless your system is very busy, it should clearly show that the job started last will finish first, as it has the lowest count.

This concludes the async example. Since AnyEvent::Fork does not actually fork, you are free to use about any module in the child, not just AnyEvent, but also IO::AIO, or Tk for example.

Example 3: Asynchronous backend with Coro

With Coro you can create a nice asynchronous backend implementation by defining an rpc server function that creates a new Coro thread for every request that calls a function "normally", i.e. the parameters from the parent process are passed to it, and any return values are returned to the parent process, e.g.:

The run function creates a new thread for every invocation, using the first argument as function name, and calls the $done callback on it's return values. This makes it quite natural to define the add and mul functions to add or multiply two numbers and return the result.

Since this is the asynchronous backend, it's quite possible to define RPC function that do I/O or wait for external events - their execution will overlap as needed.

There is an important twist - the AnyEvent::Fork::RPC::event function is only defined when the child is fully initialised. If you redirect the log messages in your init function for example, then the event function might not yet be available. This is why the log callback checks whether the fucntion is there using defined, and only then uses it to log the message.

This run function/method can be used in place of the AnyEvent::Fork::run method. Just like that method, it takes over the AnyEvent::Fork process, but instead of calling the specified $function directly, it runs a server that accepts RPC calls and handles responses.

It returns a function reference that can be used to call the function in the child process, handling serialisation and data transfers.

The following key/value pairs are allowed. It is recommended to have at least an on_error or on_event handler set.

on_error => $cb->($msg)

Called on (fatal) errors, with a descriptive (hopefully) message. If this callback is not provided, but on_event is, then the on_event callback is called with the first argument being the string error, followed by the error message.

If neither handler is provided, then the error is reported with loglevel error via AE::log.

on_event => $cb->(...)

Called for every call to the AnyEvent::Fork::RPC::event function in the child, with the arguments of that function passed to the callback.

Also called on errors when no on_error handler is provided.

on_destroy => $cb->()

Called when the $rpc object has been destroyed and all requests have been successfully handled. This is useful when you queue some requests and want the child to go away after it has handled them. The problem is that the parent must not exit either until all requests have been handled, and this can be accomplished by waiting for this callback.

init => $function (default none)

When specified (by name), this function is called in the child as the very first thing when taking over the process, with all the arguments normally passed to the AnyEvent::Fork::run function, except the communications socket.

It can be used to do one-time things in the child such as storing passed parameters or opening database connections.

It is called very early - before the serialisers are created or the $function name is resolved into a function reference, so it could be used to load any modules that provide the serialiser or function. It can not, however, create events.

done => $function (default CORE::exit)

The function to call when the asynchronous backend detects an end of file condition when reading from the communications socket and there are no outstanding requests. It's ignored by the synchronous backend.

By overriding this you can prolong the life of a RPC process after e.g. the parent has exited by running the event loop in the provided function (or simply calling it, for example, when your child process uses EV you could provide EV::run as done function).

Of course, in that case you are responsible for exiting at the appropriate time and not returning from

async => $boolean (default: 0)

The default server used in the child does all I/O blockingly, and only allows a single RPC call to execute concurrently.

Setting async to a true value switches to another implementation that uses AnyEvent in the child and allows multiple concurrent RPC calls (it does not support recursion in the event loop however, blocking condvar calls will fail).

The actual API in the child is documented in the section that describes the calling semantics of the returned $rpc function.

If you want to pre-load the actual back-end modules to enable memory sharing, then you should load AnyEvent::Fork::RPC::Sync for synchronous, and AnyEvent::Fork::RPC::Async for asynchronous mode.

If you use a template process and want to fork both sync and async children, then it is permissible to load both modules.

All arguments, result data and event data have to be serialised to be transferred between the processes. For this, they have to be frozen and thawed in both parent and child processes.

By default, only octet strings can be passed between the processes, which is reasonably fast and efficient and requires no extra modules (the AnyEvent::Fork::RPC distribution does not provide these extra serialiser modules).

For more complicated use cases, you can provide your own freeze and thaw functions, by specifying a string with perl source code. It's supposed to return two code references when evaluated: the first receives a list of perl values and must return an octet string. The second receives the octet string and must return the original list of values.

If you need an external module for serialisation, then you can either pre-load it into your AnyEvent::Fork process, or you can add a use or require statement into the serialiser string. Or both.

Here are some examples - all of them are also available as global variables that make them easier to use.

$AnyEvent::Fork::RPC::STRING_SERIALISER - octet strings only

This serialiser (currently the default) concatenates length-prefixes octet strings, and is the default. That means you can only pass (and return) strings containing character codes 0-255.

The main advantages of this serialiser are the high speed and that it doesn't need another module. The main disadvantage is that you are very limited in what you can pass - only octet strings.

This serialiser creates CBOR::XS arrays - you have to make sure the CBOR::XS module is installed for this serialiser to work. It can be beneficial for sharing when you preload the CBOR::XS module in a template process.

CBOR::XS is about as fast as the octet string serialiser, but supports complex data structures (similar to JSON) and is faster than any of the other serialisers. If you have the CBOR::XS module available, it's the best choice.

This serialiser creates JSON arrays - you have to make sure the JSON module is installed for this serialiser to work. It can be beneficial for sharing when you preload the JSON module in a template process.

JSON (with JSON::XS installed) is slower than the octet string serialiser, but usually much faster than Storable, unless big chunks of binary data need to be transferred.

This serialiser uses Storable, which means it has high chance of serialising just about anything you throw at it, at the cost of having very high overhead per operation. It also comes with perl. It should be used when you need to serialise complex data structures.

This serialiser also uses Storable, but uses it's "network" format to serialise data, which makes it possible to talk to different perl binaries (for example, when talking to a process created with AnyEvent::Fork::Remote).

See the examples section earlier in this document for some actual examples.

$rpc->(..., $cb->(...))

The RPC object returned by AnyEvent::Fork::RPC::run is actually a code reference. There are two things you can do with it: call it, and let it go out of scope (let it get destroyed).

If async was false when $rpc was created (the default), then, if you call $rpc, the $function is invoked with all arguments passed to $rpc except the last one (the callback). When the function returns, the callback will be invoked with all the return values.

If async was true, then the $function receives an additional initial argument, the result callback. In this case, returning from $function does nothing - the function only counts as "done" when the result callback is called, and any arguments passed to it are considered the return values. This makes it possible to "return" from event handlers or e.g. Coro threads.

The other thing that can be done with the RPC object is to destroy it. In this case, the child process will execute all remaining RPC calls, report their results, and then exit.

See the examples section earlier in this document for some actual examples.

CHILD PROCESS USAGE

The following function is not available in this module. They are only available in the namespace of this module when the child is running, without having to load any extra modules. They are part of the child-side API of AnyEvent::Fork::RPC.

AnyEvent::Fork::RPC::event ...

Send an event to the parent. Events are a bit like RPC calls made by the child process to the parent, except that there is no notion of return values.

See the examples section earlier in this document for some actual examples.

Note: the event data, like any data send to the parent, might not be sent immediatelly but queued for later sending, so there is no guarantee that the event has been sent to the parent when the call returns - when you e.g. exit directly after calling this function, the parent might never receive the event.

PROCESS EXIT

If and when the child process exits depends on the backend and configuration. Apart from explicit exits (e.g. by calling exit) or runtime conditions (uncaught exceptions, signals etc.), the backends exit under these conditions:

Synchronous Backend

The synchronous backend is very simple: when the process waits for another request to arrive and the writing side (usually in the parent) is closed, it will exit normally, i.e. as if your main program reached the end of the file.

That means that if your parent process exits, the RPC process will usually exit as well, either because it is idle anyway, or because it executes a request. In the latter case, you will likely get an error when the RPc process tries to send the results to the parent (because agruably, you shouldn't exit your parent while there are still outstanding requests).

The process is usually quiescent when it happens, so it should rarely be a problem, and END handlers can be used to clean up.

Asynchronous Backend

For the asynchronous backend, things are more complicated: Whenever it listens for another request by the parent, it might detect that the socket was closed (e.g. because the parent exited). It will sotp listening for new requests and instead try to write out any remaining data (if any) or simply check whether the socket can be written to. After this, the RPC process is effectively done - no new requests are incoming, no outstanding request data can be written back.

Since chances are high that there are event watchers that the RPC server knows nothing about (why else would one use the async backend if not for the ability to register watchers?), the event loop would often happily continue.

This is why the asynchronous backend explicitly calls CORE::exit when it is done (under other circumstances, such as when there is an I/O error and there is outstanding data to write, it will log a fatal message via AnyEvent::Log, also causing the program to exit).

You can override this by specifying a function name to call via the done parameter instead.

ADVANCED TOPICS

Choosing a backend

So how do you decide which backend to use? Well, that's your problem to solve, but here are some thoughts on the matter:

Synchronous

The synchronous backend does not rely on any external modules (well, except common::sense, which works around a bug in how perl's warning system works). This keeps the process very small, for example, on my system, an empty perl interpreter uses 1492kB RSS, which becomes 2020kB after use warnings; use strict (for people who grew up with C64s around them this is probably shocking every single time they see it). The worker process in the first example in this document uses 1792kB.

Since the calls are done synchronously, slow jobs will keep newer jobs from executing.

The synchronous backend also has no overhead due to running an event loop - reading requests is therefore very efficient, while writing responses is less so, as every response results in a write syscall.

If the parent process is busy and a bit slow reading responses, the child waits instead of processing further requests. This also limits the amount of memory needed for buffering, as never more than one response has to be buffered.

The API in the child is simple - you just have to define a function that does something and returns something.

It's hard to use modules or code that relies on an event loop, as the child cannot execute anything while it waits for more input.

Asynchronous

The asynchronous backend relies on AnyEvent, which tries to be small, but still comes at a price: On my system, the worker from example 1a uses 3420kB RSS (for AnyEvent, which loads EV, which needs XSLoader which in turn loads a lot of other modules such as warnings, strict, vars, Exporter...).

It batches requests and responses reasonably efficiently, doing only as few reads and writes as needed, but needs to poll for events via the event loop.

Responses are queued when the parent process is busy. This means the child can continue to execute any queued requests. It also means that a child might queue a lot of responses in memory when it generates them and the parent process is slow accepting them.

The API is not a straightforward RPC pattern - you have to call a "done" callback to pass return values and signal completion. Also, more importantly, the API starts jobs as fast as possible - when 1000 jobs are queued and the jobs are slow, they will all run concurrently. The child must implement some queueing/limiting mechanism if this causes problems. Alternatively, the parent could limit the amount of rpc calls that are outstanding.

Blocking use of condvars is not supported (in the main thread, outside of e.g. Coro threads).

Using event-based modules such as IO::AIO, Gtk2, Tk and so on is easy.

Passing file descriptors

The reason is that passing file descriptors is extraordinary tricky business, and conflicts with efficient batching of messages.

There still is a method you can use: Create a AnyEvent::Util::portable_socketpair and send_fh one half of it to the process before you pass control to AnyEvent::Fork::RPC::run.

Whenever you want to pass a file descriptor, send an rpc request to the child process (so it expects the descriptor), then send it over the other half of the socketpair. The child should fetch the descriptor from the half it has passed earlier.

Of course, this might be blocking if you pass a lot of file descriptors, so you might want to look into AnyEvent::FDpasser which can handle the gory details.

EXCEPTIONS

There are no provisions whatsoever for catching exceptions at this time - in the child, exceptions might kill the process, causing calls to be lost and the parent encountering a fatal error. In the parent, exceptions in the result callback will not be caught and cause undefined behaviour.

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